For many years asbestos has been used in combination with asphalt to produce a variety of coating compositions, such as roof coatings and roof cements. The asphalt is used as a binder and the asbestos is used for texture, strength, and the thixotropic properties which it imparts to the formulation.
Because of toxicity problems and the fact that asbestos may be a carcinogen, asbestos has fallen out or favor and in some places has been banned. For many types of products, including asphalt-based coatings, substitutes for asbestos have been found or developed.
Finding suitable substitute materials for asbestos for use in asphalt-based coatings has been a significant concern in the art. One approach which has been used with good success has involved combining clay, a quaternary ammonium salt surfactant composition, and fillers, together with an asphalt cutback.
The clay is for the purpose of achieving the gelling properties earlier provided by the traditionally-used asbestos. But clays have had to be used in conjunction with the quaternary ammonium salt cationic surfactants in order to develop optimum wetting and dispersion. Reducing the interfacial tension of the clay with the asphalt allows formation of a clay-asphalt dispersion to provide the thickened system. Such clay-thickened asphalt cutbacks, while developed primarily to overcome asbestos concerns, have shown some performance characteristics that exceed those shown by the previous asbestos-containing compositions.
The art had come to accept that such quaternary ammonium salts were the cationic surfactants necessary for effective use in such non-asbestos asphalt-based coating formulations. The most effective of these quaternary ammonium salt surfactants have been a small number of quaternary ammonium chlorides, such as the dialkyl quaternary ammonium chlorides and monoalkyl quaternary ammonium chlorides. Examples of these include Arquad 2C-75 (dicoco dimethyl ammonium chloride), Arquad T-50 (tallow trimethyl ammonium chloride), and 2HT-75 (dihydrogenated dimethyl ammonium chloride), all sold by Akzo Chemie Americas, of Chicago, Ill.
The quaternary ammonium chloride salt surfactants of asbestos-free asphalt-based coatings of the prior art, particularly the widely-used dialkyl quaternary ammonium chlorides, have a number of significant problems and drawbacks.
One significant problem is that, at room temperature such substances are solid or paste, which makes handling and blending difficult, so that typically they are solubilized into liquid surfactant compositions which are less than 100% active. The solvents present also raise concern about increased flammability hazards during the blending process.
Another significant concern relates to corrosivity. The widely-used quaternary ammonium chlorides can pose corrosion risks, particularly when the asphalt-based coatings containing such quaternaries are packaged and stored in metal containers or otherwise come into contact with corrosion-prone metals.
A very significant drawback is the considerable cost which use of the quaternary ammonium chloride salt surfactant compositions add to the cost of the roof-coating compositions. This cost relates to the relatively large amounts of such surfactant compositions which are required in order to provide asphalt based coating compositions with adequate gel qualities--that is, sufficient gel strength and thixotropy and, of equal importance, sufficient stability in gel qualities over time.
Formulations of the prior art, after addition of the quaternary surfactant composition and clay, are typically close to 90% asphalt cutback by weight, and the weight ratio of the clay to the surfactant composition generally varies from about 4:1 to about 7:1, depending on formulation variables. In comparison with the amounts of the other constituents used in these asphalt-based coating formulations, such as asphalt cutback, clay and fillers, the amount of surfactant used is small. However, the high relative cost of the surfactant composition (compared to the cost of the clay and asphalt cutback) causes even the small amount used to have an important impact on the cost of the total formulation.
Raising the ratio of clay to surfactant composition thus has the effect or reducing formulation cost. The amount of surfactant composition required depends on various factors, including the asphalt cutback chosen and the specific quaternary ammonium salt chosen. For any type of formulation, however, the right amount of surfactant composition must be present to provide good gel structure and stability.
In summary, a number of drawbacks and problems exist in the art relating to asbestos-free asphalt-based coatings. There is a need for an improved asbestos free formulation.